This invention relates to a light beam scanning apparatus for scanning and exposing a single photo-sensitive drum simultaneously with laser beams to form an electrostatic latent image on the drum and to an image forming apparatus, such as a digital copying machine or a laser printer, using the light beam scanning apparatus.
In recent years, digital copying machines have been developed which form images by laser-beam scanning and exposure and electronic photographic processing.
To step up the image forming speed, a multi-beam digital copying machine has also been developed. In this type of copying machine, multiple laser beams are generated and caused to make simultaneous scanning in units of more than one line.
The multi-beam digital copying machine includes an optical beam scanning apparatus. The optical beam scanning apparatus comprises semiconductor laser oscillators for generating laser beams, a multiplanar rotating mirror, such as a polygon mirror, for reflecting the laser beams from the laser oscillators toward the photosensitive drum and causing each laser beam to scan the drum, and an optical unit mainly composed of a collimator lens and an f-xcex8 lens.
With the configuration of the conventional optical system unit, however, it is very difficult to ideally adjust the positional relationship between the light beams on the photosensitive drum (a surface to be scanned). Realizing this requires very high accuracy of component parts and assembly, which results in a rise in the manufacturing cost of the apparatus.
Even if the apparatus has been assembled in the ideal positional relationship, environmental changes, including temperature changes and humidity changes, slight changes in the shape of the lens with time, or slight changes in the positional relationship between the component parts will ruin the positional relationship between the light beams, making it impossible to form a high-quality image. Therefore, to realize the ideal optical system needs a configuration and components parts immune to those changes.
The conditions for obtaining a high-quality image by use of the multi-beam optical system are as follows:
(1) The optical power of each beam on the photosensitive drum has to be the same.
(2) The light beams have to have a specific positional relationship (the passing positions of the light beams have to have a specific positional relationship in the sub-scanning direction).
(3) The exposure timing to form an image in the main scanning direction has to be controlled according to the positional relationship between the light beams.
Of these conditions, to fulfill condition (1) is the most important. The reason is that if the optical power is not the same on the photosensitive drum, the positions will not be adjusted correctly in a true sense even when condition (2) and condition (3) appear to have been fulfilled correctly. Namely, the accuracy of control in condition (2) and condition (3) depends on the power of each light beam.
Therefore, each light beam scanning the photo-sensitive drum, that is, each light beam emitted from each of the laser oscillators, has to have the same power after it has been combined with another by means of optical means, such as a half mirror.
An object of the present invention is to provide a light beam scanning apparatus capable of controlling the power of a light beam at a surface to be scanned to a desired value, constantly controlling the passing position of the light beam on the surface and the exposure timing suitably with high accuracy, and therefore, always assuring high picture quality, and an image forming apparatus using the optical beam scanning apparatus.
Another object of the present invention is to provide a light beam scanning apparatus capable of controlling the power of each light beam at a surface to be scanned to a desired value in using more than one light beam, constantly controlling the passing position of each light beam to the ideal position and the exposure timing of each light beam with high accuracy even when the positional relationship between light beam on the surface is caused to correspond to more than one resolution, and therefore always assuring high picture quality, and an image forming apparatus using the light beam scanning apparatus.
In order to achieve to the above objects, according to one aspect of the present invention, there is provided a light beam scanning apparatus comprising: a plural light beam generating means for generating a light beam; plural optical means, provided to each of the plural light beam generating means respectively, for combining the plural light beams; single scanning means for reflecting the light beams combined by the combining means toward a surface to be scanned and causing the light beams to scan the surface in a main scanning direction; light beam power sensing means for using a first light-receiving element to sense the power of each of the light beams caused by the scanning means to scan the surface; and light beam power control means for controlling each of the plural light beam generating means on the basis of the result of sensing at the light beam power sensing means so that the difference between the power of one of the light beams scanning the surface and the power of each of the remaining three may be equal to or less than a specific value.
With the scanning apparatus that uses light beams to scan the photosensitive drum, the power of each light beam on the surface to be scanned or the surface of the photosensitive drum is controlled to a desired value.
A sync clock is generated after a delay from when each light beam moves across a light beam detector. The spacing between each light receiving element is set to an integral multiple of the length of one dot formed by a light beam on the photosensitive drum in the main scanning direction. In synchronism with the synch clocks, pixel clocks and image data are applied to laser oscillators to form an image on the surface of the photosensitive drum. The synch clock generating operation are performed after the light beams are controlled to have substantially the same power.
According to another aspect of the present invention, there is provided a light beam scanning apparatus comprising: plural light beam generating means for each generating light beams; scanning means for reflecting the light beams generated at the plural light beam generating means toward a surface to be scanned and causing the light beams to scan the surface in the main scanning direction; first light beam position sensing means for using light-receiving elements arranged linearly at intervals corresponding to a first resolution in the sub-scanning direction perpendicular to the main scanning direction to sense the light beams caused by the scanning means to scan the surface; second light beam position sensing means for using light-receiving elements arranged linearly at intervals corresponding to a second resolution differing from the first resolution in the sub-scanning direction to sense the light beams caused by the scanning means to scan the surface; and light beam passing position control means for controlling the light beams caused to scan by the scanning means on the basis of the results of sensing at the first and second light beam position sensing means so that each of the passing positions of the light beams on the surface may become a proper position according to the first or second resolution.
With the scanning apparatus that uses light beams to scan the surface to be scanned, even when more than one resolution is used, the light beams on the surface to be scanned are constantly controlled with high accuracy so as to have an ideal positional relationship between them.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.